This study will study subjects with panic disorder and healthy controls. Based on previous studies the following hypotheses will be tested (2 replication and 2 new hypotheses): Replication; (1) panic disorder subjects will report higher anxiety after a caffeine challenge than the healthy control subjects. (2) healthy controls with the1976 T/T polymorphism will report increased anxiety after a caffeine challenge compared to healthy controls with the 1976 C/T and 1976 C/C genotypes, New hypotheses; (3) panic patients (two separate groups: currently ill and remitted) with the 1976 T/T polymorphism will report increased anxiety after a caffeine challenge compared to panic patients with the 1976 C/T and 1976 C/C genotypes, (4) panic patients (two separate groups: currently ill and remitted) with the 1976 T/T polymorphism will report increased anxiety after a caffeine challenge compared to healthy controls with the 1976 T/T polymorphism will report increased anxiety after a caffeine challenge compared to healthy controls with the 1976 T/T genotype.